1. Field of the Invention
The present invention relates generally to a communication system, and in particular, the present invention relates to an apparatus and method for handover in a communication system.
2. Description of the Related Art
The next generation communication system has evolved to enable the system to perform high-speed data transmission/reception while ensuring the mobility of a plurality of Mobile Stations (MSs). The next generation communication system can provide services having various Quality of Service (QoS) classes. An Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system is an example of the next generation communication system.
In the IEEE 802.16e communication systems, handover is a very important factor to ensure the mobility of the MSs. The term ‘handover’ as used herein refers to an action of allowing an MS in service to receive a seamless service while moving between Base Stations (BSs). The ‘handover’ refers to an action in which an MS moves to a target BS from which it can receive a higher-quality service than its serving BS from which it is presently receiving a service, in response to a decrease in the quality of the service being provided from the serving BS, thereby receiving the seamless service.
FIG. 1 illustrates a handover procedure in an IEEE 802.16e communication system.
Referring to FIG. 1, a serving BS 102 transmits a Mobile Neighbor Advertisement (MOB_NBR-ADV) message to an MS 100 in step 101. Upon receipt of the MOB_NBR-ADV message, the MS 100 can acquire information on neighbor BSs from the received MOB_NBR-ADV message. The MS 100 scans Carrier-to-Interference and Noise Ratios (CINRs), based on which it can measure signal qualities of reference signals, or pilot signals, transmitted from the neighbor BSs in step 103.
After scanning CINRs received from neighbor BS 104 and neighbor BS 106, when the MS 100 determines to switch from its current serving BS 102 to a new BS, it transmits a Mobile Station Handover Request (MOB_MSHO-REQ) message to the serving BS 102 in step 105.
Upon receipt of the MOB_MSHO-REQ message, the serving BS 102 detects a recommended neighbor BS list of the neighbor BSs to which the MS 100 can perform handover, from an Information Element (IE) of the MOB_MSHO-REQ message. The recommended neighbor BS list includes information on the recommended neighbor BSs to which the MS 100 prefers to perform handover. For convenience, it is assumed in FIG. 1 that the recommended neighbor BS list includes the neighbor BS1 104 and the neighbor BS2 106.
The serving BS 102 transmits HO_notification messages to the recommended neighbor BSs, i.e. the neighbor BS1 104 and the neighbor BS2 106, included in the recommended neighbor BS list in steps 107 and 109. The HO_notification message includes information on the bandwidth and QoS that a new serving BS, or an expected target BS, of the MS 100 should provide. Upon receipt of the HO_notification messages, the neighbor BS1 104 and neighbor BS2 106 transmit HO_notification-RSP messages to the serving BS 102 in response to the HO_notification messages in steps 111 and 113. The HO_notification-RSP message includes an action time required until the MS 100 is assigned to a dedicated transmission opportunity for transmitting of a Ranging Request (RNG-REQ) message in performing a network reentry procedure after performing handover to the target BS. Further, the HO_notification-RSP message includes QoS information supportable in the neighbor BS 104 and the neighbor BS 106 among the QoSs used by the MS 100.
The serving BS 102 receives the HO_notification-RSP messages from the neighbor BS1 104 and the neighbor BS2 106 in steps 111 and 113. The serving BS 102 includes information of the neighbor BSs, for example, the action time and QoS information, acquired from the HO_notification-RSP messages, in a Mobile BS HandOver Response (MOB_BSHO-RSP) message, and transmits it to the MS 100 in step 115.
Upon receipt of the MOB_BSHO-RSP message, the MS 100 determines a final target BS that can provide the bandwidth and QoS requested by the MS 100. It will be assumed herein that the MS 100 has determined the neighbor BS1 104 as a final target BS. After determining the final target BS, the MS 100 transmits a Mobile Handover Indication (MOB_HO-IND) message to the serving BS 102 in response to the MOB_BSHO-RSP message in step 117. Upon receipt of the MOB_HO-IND message, the serving BS 102 transmits a HO_confirm message to the target BS 104 included in the MOB_HO-IND message in step 119.
Although not illustrated in the drawing, after transmitting the MOB_HO-IND message to the serving BS 102 in step 117, the MS 100 stops the communication with the serving BS 102 and acquires synchronization with the target BS 104. After acquiring the synchronization with the target BS 104, the MS 100 receives a downlink MAP (DL-MAP) and an uplink MAP (UL-MAP) from the target BS 104 and then attempts handover ranging.
After the MS 100 transmits the MOB_HO-IND message to the serving BS 102, when the serving BS 102 needs a time in analyzing and processing the MOB_HO-IND message, the serving BS 102 can transmit downlink data to the MS 100 while analyzing and processing the corresponding MOB_HO-IND message. In this case, because the MS 100 may interrupt the communication path to the serving BS 102 immediately after transmitting the MOB_HO-IND message to the serving BS 102, the downlink data transmitted from the serving BS 102 to the MS 100 may be lost.
Given that the serving BS 102 needs a time in processing the MOB_HO-IND message after the MS 100 transmits the MOB_HO-IND message to the serving BS 102, the MS 100 may delay the communication path interruption to the serving BS 102, causing a handover delay.
As described above, in the handover procedure of the current IEEE 802.16e communication system, the time at which the MS interrupts the communication path to the serving BS and attempts an access to the target BS after transmitting the MOB_HO-IND message to the serving BS has not been clearly defined.